Today's drives of this type usually have, in the power supply, energy-storing cells such as batteries that are understood in particular to be rechargeable, such as lithium-polymer batteries. In addition, pulse inverters are usually included in such a drive.
In this case, the energy-storing cells perform the task of providing the energy required for operation of the drive, in particular moving the vehicle, and/or storing the energy during charging. The pulse inverter converts the DC voltage provided by the battery into a typically three-phase AC voltage with which an electric motor, such as a synchronous or asynchronous machine, is then operated via power electronics that control the stator windings.
The energy-storing cells, pulse inverters and power electronics are usually manufactured independently of each other, and form independent units that are connected to each other by wire harnesses. In this case, a suitable compromise must always be found as part of the system design between the magnitude of the currents flowing in the system and the voltage level.
For a drive with a power of, for example, 100 kW the battery could either be designed with a DC voltage of 100V and an output current of about 1000 A, or with higher voltages and accordingly lower currents.
For example, in the field of application of today's electric vehicles, a voltage of about 400-600V has prevailed for the time being, resulting in currents in the range of a few hundred amperes. Low voltages and higher currents are not feasible to implement in drives to-date, since the cross-sections of the current-carrying cables and motor windings would need to increase massively, which would lead to an increase in vehicle weight and cost.
The voltage level of, typically, >400V furthermore leads in the prior art to considerable demands in terms of electrical safety of such systems, and introduces considerable difficulty regarding insulation of the individual components of the vehicle chassis and the corresponding insulation monitoring.
These expenses could be reduced only once voltages drop to <60V, as far as VDE standards are concerned. However, the significantly increased currents required in this case cannot be handled in a cost-efficient manner in the drives to-date as a result of distances that must be covered and the required conductor cross-sections.
An essential criterion in the design of an electric or hybrid drive for a vehicle in such a case is the volumetric energy and/or power density, that is, the volume of the electric drive train based on the energy content (that constitutes a measure of the range of the vehicle) and/or based on the weight of the power train.